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United States Patent |
5,270,258
|
Yoshida
|
December 14, 1993
|
Microminiature vacuum tube manufacturing method
Abstract
A method of manufacturing a microminiature vacuum tube includes forming a
mask layer on a first surface of a monocrystalline substrate and removing
the mask layer where a cathode is to be formed, anisotropically etching
the monocrystalline substrate at the surface using the mask layer to form
a recess in the substrate having a V-shaped cross-section, covering the
V-shaped recess with an electron-emitting cathode material, successively
forming a first insulator film, a gate material, a second insulator film,
and an anode material on the second surface of the substrate, removing
portions of the anode material, second insulator film, gate material, and
first insulator film from a portion of the second surface opposite the
V-shaped recess, and etching the monocrystalline substrate using the first
insulator film as a mask until the tip of the cathode material is exposed.
Uniformly shaped cathodes can be formed with good controllability and
reproducibility according to the invention.
Inventors:
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Yoshida; Masahiro (Itami, JP)
|
Assignee:
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Mitsubishi Denki Kabushiki Kaisha (Tokyo, JP)
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Appl. No.:
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720611 |
Filed:
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June 25, 1991 |
Foreign Application Priority Data
Current U.S. Class: |
438/20; 438/928; 445/50; 445/51 |
Intern'l Class: |
H01L 021/465 |
Field of Search: |
437/93,228,203
445/41-52
313/309,313,351,336
156/647
|
References Cited
U.S. Patent Documents
4149175 | Apr., 1979 | Inoue et al. | 437/129.
|
4983878 | Jan., 1991 | Lee et al. | 313/336.
|
5012153 | Apr., 1991 | Atkinson et al. | 313/309.
|
5090932 | Feb., 1992 | Dieumegard et al. | 437/93.
|
5126287 | Jun., 1992 | Jones | 445/50.
|
Foreign Patent Documents |
0272178 | Jun., 1988 | EP.
| |
0278405 | Aug., 1988 | EP.
| |
0306173 | Mar., 1989 | EP.
| |
56-160740 | Dec., 1981 | JP | 437/228.
|
8806345 | Aug., 1988 | WO.
| |
2209432 | May., 1989 | GB.
| |
2240426 | Jul., 1991 | GB.
| |
Other References
Spindt et al., "Physical Properties of Thin-Film Field Emission Cathodes
With Molybdenum Cones", Journal of Applied Physics, vol. 47, No. 12, Dec.
1976, pp. 5248-5263.
|
Primary Examiner: Chaudhuri; Olik
Assistant Examiner: Horton; Ken
Attorney, Agent or Firm: Leydig, Voit & Mayer
Claims
What is claimed is:
1. A method of manufacturing a microminiature vacuum tube comprising the
sequential steps of:
forming a mask layer on a first surface of a monocrystalline substrate and
removing the mask layer where a cathode is to be formed;
anisotropically etching said monocrystalline substrate at the first surface
using said mask layer as a mask, thereby forming a recess in said
substrate having a V-shaped cross-section;
covering said V-shaped recess with an electron-emitting cathode material;
forming a first insulator film on a second surface of said monocrystalline
substrate opposite the first surface, forming a gate material on said
first insulator film, forming a second insulator film on said gate
material, and forming an anode material on said second insulator film;
removing portions of said anode material, second insulator film, gate
material, and first insulator film to expose the second surface of said
substrate opposite the V-shaped recess in said monocrystalline substrate;
and
etching said monocrystalline substrate at the second surface using said
anode, second insulator, gate, and first insulator films as a mask to
expose said cathode material.
2. A method of manufacturing a microminiature vacuum tube in accordance
with claim 1, wherein said monocrystalline substrate has a dependency on
crystal orientation in etching.
3. A method of manufacturing a microminiature vacuum tube in accordance
with claim 1, wherein said monocrystalline substrate material is silicon
having (100) oriented first and second surfaces including anisotropically
etching said silicon substrate with a mixture of potassium hydroxide and
isopropyl alcohol to expose said cathode material, said V-shaped recess
having (111) oriented surfaces.
4. A method of manufacturing a microminiature vacuum tube in accordance
with claim 1, wherein said monocrystalline substrate material is GaAs
having (100) oriented first and second surfaces including anisotropically
etching said substrate with a mixture of sulfuric acid, hydrogen peroxide,
and water to expose said cathode material.
5. A method of manufacturing a microminiature vacuum tube in accordance
with claim 1, including depositing one of SiO.sub.2, Si.sub.3 N.sub.4, and
SiNO as said mask layer.
Description
FIELD OF THE INVENTION
The present invention relates to a microminiature vacuum tube having a
cathode which emits electrons by means of electric field emission, a gate
which controls the electrons and an anode which receives the electrons,
and housed in a vacuum container. The present invention also relates to a
manufacturing method thereof.
BACKGROUND OF THE INVENTION
The microminiature vacuum tube utilizes electrons traveling in vacuum, and
unlike the general vacuum tubes, it is formed on a semiconductor
substrate. Therefore, a cathode of electric field emission type is used
which emits electrons by means of an electric field. To emit electrons,
the shape of the electron emitting end of the cathode is required to be as
sharp as possible.
A description is given of an example of the conventional method of
manufacturing a microminiature vacuum tube with reference to FIGS.
3(a)-3(e).
First, as shown in FIG. 3(a), a mask material is formed on the entire
surface of a monocrystalline substrate 1, and the mask material on
portions other than a portion 2 to become a cathode is removed by
photolithography.
Next, as shown in FIG. 3(b), the substrate 1 is etched by dry etching such
as RIE (reactive ion etching) using the mask material 2 as a mask.
Furthermore, the substrate 1 is etched in the lateral direction and
obliquely by anisotropic wet etching using an etchant such as potassium
hydroxide, and a protrusion is formed which has an acute-angled tip 9
which becomes a cathode later (FIG. 3(c)).
Next, an insulating material 5 for protecting the tip shape of the cathode
is formed on the entire surface of the substrate and a metal film 68 is
formed thereon, and thereafter resist patterns 11 are produced thereon by
photolithography (FIG. 3(d)). The metal film 68 and the insulating
material 5 are etched by RIE or the like using resist patterns 11 as a
mask and a gate 6 and an anode 8 are at periphery of the cathode formed on
the substrate 1, thereby completing a device (FIG. 3(e)).
When this device is used, the cathode voltage Vc is made the ground level
by grounding the substrate 1 as shown in FIG. 4, and a voltage V.sub.A of
100 to 500 V is applied to the anode 8. Electrons emitted from the cathode
9 into vacuum by means of electric field emission are collected by the
anode 8. Meanwhile, the quantity of electrons flowing from the cathode 9
to the anode 8 is controlled by applying a voltage of several tens of
volts to the gate 6 as a gate voltage V.sub.G.
In the conventional microminiature vacuum tube manufactured by the method
as described above, etching in the lateral direction is utilized to form
the cathode, therefore the control of timing for ending etching when the
tip shape of the cathode becomes acute-angled is very difficult.
Particularly, in fabricating a plurality of cathodes on the substrate,
this control is further difficult. Actually, as shown in FIG. 5, a cathode
12b which has not been etched fully, a cathode 12c which has been etched
excessively and the like are formed besides a cathode 12a having a desired
shape. Thus, variations occur in the shape of the cathode.
Also, the area of adhesion between the portion to become the cathode on the
surface of the substrate 1 and the mask material 2 becomes smaller as the
etching progresses, and therefore the adhesion force between the both is
weakened. This results in peeling of the mask material and the etched
shape varies. Therefore it is difficult to obtain a uniform etched shape.
Further, the tip of the cathode is required to be protected when the gate
and the anode are formed, and in the conventional example, the tip is
protected by an insulator film such as SiO.sub.2. However, the tip part of
the cathode is actually exposed to an etching gas immediately before the
gate 6 and the anode 8 are formed, and for this reason, the tip part of
the cathode is damaged and it is difficult to maintain the original sharp
tip shape.
As described above, in the conventional manufacturing method, the
controllability and the reproducibility of the etching process for forming
the cathode are worse, and further the tip part of the cathode is damaged
in the stage of forming the gate and the anode, incurring non-uniformity
in the device characteristics.
SUMMARY OF THE INVENTION
The present invention is directed to solving the above-described problems
and has its object to provide a microminiature vacuum tube which can
produce a cathode shape with good uniformity and which can be easily
integrated. Another object of the present invention is to provide a
manufacturing method of a microminiature vacuum tube.
A manufacturing method of a microminiature vacuum tube in accordance with
the present invention comprises the following steps:
(a) forming a mask layer on a monocrystalline substrate, and removing a
portion of the mask layer, where a cathode is to be formed, by
photolithography,
(b) etching the monocrystalline substrate with the mask layer used as a
mask using an anisotropic etchant, producing a recess having a V-shaped
cross-section and forming a material to become the cathode in the recess,
(c) forming a first insulating material on the surface opposite the recess
of the monocrystalline substrate, forming a material to become a gate,
forming a second insulating material on the top surface thereof, and
further forming a material to become an anode on the top surface thereof,
(d) removing the anode material, the insulator film and the gate material
on the portion facing the cathode tip by photolithography,
(e) etching the monocrystalline substrate with the gate material used as a
mask until the tip of the cathode material appears.
In the microminiature vacuum tube in accordance with the present invention,
the tip part of the cathode material manufactured by the above-mentioned
processes (a) through (e) becomes the cathode, and the gate material and
the anode material remaining in the above-mentioned process (d) become the
gate and the anode.
In the method of manufacturing the microminiature vacuum tube of the
present invention, since only anisotropic etching of monocrystalline is
used as a means for forming the shape of the cathode, the shape of the tip
is obtained stably.
Since the tip portion of the cathode is protected by the material of the
substrate until the gate and the anode are completed formed, changes in
the shape of the cathode tip do not occur in manufacturing.
In the microminiature vacuum tube of the present invention, the gate and
the anode are located along a direction perpendicular to the cathode, and
therefore the interval between the cathode and the anode can be made as
small as possible in manufacturing, and integration thereof with other
devices is simplified.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(a)-1(f) are views showing a method of manufacturing a
microminiature vacuum tube in accordance with an embodiment of the present
invention;
FIG. 2 is a view for explaining operation of a microminiature vacuum tube
formed by a manufacturing method in accordance, with an embodiment of the
present invention.
FIGS. 3(a)-3(e) are views showing a conventional method of manufacturing a
microminiature vacuum tube.
FIG. 4 is a view for explaining operation of a microminiature vacuum tube
formed by the conventional manufacturing method; and
FIG. 5 is a view for explaining a problem in the conventional method of
manufacturing a microminiature vacuum tube.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, description is made on an embodiment of the present invention
in reference to drawings.
FIGS. 1(a)-1(f) are views showing respective major processes in a method of
manufacturing a microminiature vacuum tube in accordance with an
embodiment of the present invention, and FIGS. 1(a)-1(e) show
cross-sectional structures of processed devices in five stages of
manufacturing process, and FIG. 1(f) shows the cross-sectional structure
of a completed device.
In FIGS. 1(a)-1(f), reference numeral 1 designates a monocrystalline
semiconductor substrate. A mask material 2 is disposed on the
semiconductor substrate 1. A V-shaped concave part 3 is formed on a first
main surface of the substrate 1. An electric field emitting material 4 is
used to become a cathode material. Reference numerals 5, 5', 7 and 7'
designate insulating materials. Reference numeral 6' designates a gate
material and reference numeral 8' designates an anode material. Reference
numeral 6 designates a gate and reference numeral 8 designates an anode.
The cathode is formed to have a sharp tip 9.
Next, description is made on a manufacturing method.
First, a monocrystalline silicon substrate having a (100) facet is used for
the monocrystalline substrate 1, and on a first main surface thereof, a
mask material such as SiO.sub.2, Si.sub.3 N.sub.4 or SiNO is formed in a
thickness of several hundreds of angstroms or more by the plasma CVD
method. A resist-pattern (not illustrated) is provided on this mask using
photolithography techniques, and a substrate surface region whereon the
cathode is to be installed is exposed by RIE using the resist pattern as a
mask (FIG. 1(a)).
Next, the substrate 1 is etched with an anisotropic etching solution such
as potassium hydroxide and isopropyl alcohol with using the mask layer 2
as a mask.
At this time, because the etching speed of a (111) facet of Si as about 30
times as fast as that of a (100) facet, when etching is performed with
window in the mask layer 2 on the substrate having such a (100) facet, the
V-shaped recess 3 consisting of (111) facets making an angle of 54.degree.
with the (100) facet is formed (FIG. 1(b)). This method of etching using
the mask layer 2 as a mask produces high adhesiveness between the mask
layer and the substrate in comparison with the method using a resist as a
mask and the shape after etching is easily stabilized. Therefore, this
method is quite advantageous.
Next, the electric field emitting material 4 comprising a material easily
emits electrons and has a small work function such as molybdenum is
formed, for example, in a thickness of 1,000 .ANG. or more by sputtering
to cover the V-shaped recess 3 (FIG. 1(c)).
Next, a Si.sub.3 N.sub.4 film as the insulating material 5' is formed on a
second main surface opposite to the face of the V-shaped recess 3 of the
substrate 1. The gate material 6' is formed on this Si.sub.3 N.sub.4 film
5', the insulating material 7' is formed on this gate material 6', and the
anode material 8' is further formed on this insulating material 7'. Here,
the film thickness of each layer is set to 1,000 .ANG. or more, and a
metal such as Au, Ti, Ni or Al is used as the gate material 6' and the
anode material 8' (FIG. 1(d)).
Next, by means of photolithography technique, a window is opened by etching
the anode material 8', the insulating material 7', the gate material 6'
and the insulating material 5' at a region confronting the V-shaped
concave part 3 by ion milling or RIE using SF.sub.6 or CF.sub.4 gas to
expose the surface of the substrate 1 (FIG. 1(e)). The gate material 6'
and the anode material 8' remaining at this time are used later as the
gate electrode 6 and the anode electrode 8.
Next, the substrate 1 is etched using the insulating material 5 as a mask,
and the tip 9 of the electric field emitting material 4 is exposed. For
this etching, wet etching using potassium hydroxide and isopropyl alcohol
is used. Since the speed of the etching of semiconductor is generally tens
of thousands of times as fast as that of metal, the electric field
emitting material such as molybdenum is not over-etched in this etching
process, and the sharp tip 9 of the electric field emitting material is
exposed at the etching opening with good controllability and good
reproducibility. Also, the shape of the tip 9 is determined by crystalline
property of the material of the monocrystalline semiconductor used for the
substrate 1, and therefore uniform shapes are always obtained (FIG. 1(f)).
Also, the insulating material 5 serves as both an insulator for isolating
the gate electrode 6 from the substrate 1 and a mask in etching the
substrate 1. The sharp tip 9 works as the cathode for emitting electrons.
As shown in FIG. 2, electrons emitted from the tip 9 of the cathode in the
vertical direction by electric field emission are controlled by a voltage
applied to the gate 6 and flow into the anode 8.
In the conventional microminiature vacuum tube, the gate and the anode are
formed along a direction parallel to the cathode, and therefore the
interval between the cathode and the anode is kept at about 50 microns at
a minimum. However, in the microminiature vacuum tube obtained by the
manufacturing method of this embodiment, the gate 6 and the anode 8 are
formed along a direction perpendicular to the cathode 9, and therefore the
interval between the cathode 9 and the anode 8 can be set easily by the
thickness of the substrate 1, the thickness of the insulating films 5 and
7, the gate 6 and the anode 8 and the like, and this interval can be set
at 10 microns or less, and further can be set to a minute value less than
several microns.
Therefore, in the microminiature vacuum tube of this embodiment, the anode
voltage V.sub.A has only to be about 100 V and the gate voltage V.sub.G
has only to be about 10 V, and a small power source can be used. Thus,
this embodiment has a big advantage in miniaturizing the device and
reducing the size of the whole system.
In the above-illustrated embodiment, a description is given of a device
having one cathode, but a plurality of cathodes can be fabricated on the
same substrate. When the individual electrodes are not separated, they are
in a parallel connection, and thereby the current capacity can be larger.
In a portion of the substrate where the cathode is not formed, the material
of the substrate is not etched, and therefore other devices such as
transistors, diodes, resistors and the like can be integrated thereon.
While in the above-mentioned embodiment a Si monocrystalline substrate is
used for the monocrystalline substrate 1, this can be another substrate,
provided that it is a material showing anisotropy in etching. For example,
a compound semiconductor substrate such as GaAs substrate or the like can
be used.
In a case where GaAs is used as the substrate 1, when a (100) facet
substrate is used and [011] direction is taken as a direction in which the
dependency of the etching on crystal orientation appears, a V-shaped
groove making an angle of about 45.degree. with the (100) facet is formed.
For the etching, for example, a solution of sulfuric acid, hydrogen
peroxide and water is preferably used.
As described above, in the microminiature vacuum tube obtained by the
manufacturing method of this embodiment, the shape of cathode is uniform,
and the interval between the cathode and anode is small, on the order of
microns, and when integrated, high performance and high reliability are
obtained without variations in the device characteristics. Thus, this
vacuum tube can be effectively used for high-frequency devices used in the
millimeter wave band.
As described above, in accordance with the present invention, a
monocrystalline substrate is etched to form a recess having a V-shaped
cross-section, the V-shaped recess is covered with a cathode material, a
first insulator film, a gate material, a second insulator film and an
anode material are sequentially formed on a second main surface of the
monocrystalline substrate, and portions thereof confronting the V-shaped
recess of the substrate are etched until the tip of the above-mentioned
cathode material appears, and the exposed sharp tip is used as the
cathode. Therefore the cathode having a uniform shape which is determined
by the crystalline property of the substrate is obtained, and further the
sharp tip of the cathode is not exposed on the surface in forming the gate
and the anode, and therefore changes in the shape of the cathode tip are
prevented. Uniformly shaped cathodes are formed with good controllability
and good reproducibility.
Furthermore, since the interval between the cathode and the anode can be
made small, a high electron emitting efficiency is obtained and the device
can be reduced in size.
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